Break self-cycle to fix leak (106860, r=dbaron, sr=bienvenu).

xpcom/io/nsFastLoadFile.cpp

@@ -2260,11 +2260,6 @@ nsFastLoadFileUpdater::Open(nsFastLoadFileReader* aReader)
 
     // Map from reader dense, zero-based MFL_OID_TO_SHARP_INDEX(oid) to sharp
     // object offset and refcnt information in updater.
-    PRUint32 saveReadOffset;
-    rv = aReader->Tell(&saveReadOffset);
-    if (NS_FAILED(rv))
-        return rv;
-
     nsFastLoadFileReader::nsObjectMapEntry* readObjectMap =
         aReader->mFooter.mObjectMap;
     for (i = 0, n = aReader->mFooter.mNumSharpObjects; i < n; i++) {
@@ -2292,10 +2287,6 @@ nsFastLoadFileUpdater::Open(nsFastLoadFileReader* aReader)
                                             readEntry);
     }
 
-    rv = aReader->Seek(nsISeekableStream::NS_SEEK_SET, saveReadOffset);
-    if (NS_FAILED(rv))
-        return rv;
-
     // Copy URI spec string and initial segment offset in FastLoad file from
     // nsDocumentMapReadEntry in reader to mDocumentMapWriteEntry in updater.
     // If we didn't enumerate all entries, we ran out of memory.
@@ -2340,11 +2331,26 @@ nsFastLoadFileUpdater::Open(nsFastLoadFileReader* aReader)
     if (NS_FAILED(rv))
         return rv;
 
+    // Avoid creating yet another object by implementing nsIFastLoadFileIO on
+    // this updater, and save aReader's input stream so it can be returned by
+    // GetInputStream called from nsFastLoadFileWriter::Close.  This requires
+    // that we override Close to break the resulting zero-length cycle.
     mFileIO = this;
     mInputStream = aReader->mInputStream;
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsFastLoadFileUpdater::Close()
+{
+    // Call base-class Close implementation, which uses mFileIO.
+    nsresult rv = nsFastLoadFileWriter::Close();
+
+    // Break degenerate cycle from this->mFileIO to this.
+    mFileIO = nsnull;
+    return rv;
+}
+
 NS_COM nsresult
 NS_NewFastLoadFileUpdater(nsIObjectOutputStream* *aResult,
                           nsIOutputStream* aOutputStream,

xpcom/io/nsFastLoadFile.h

@@ -445,7 +445,7 @@ class NS_COM nsFastLoadFileWriter
 
     nsresult   Init();
     nsresult   Open();
-    NS_IMETHOD Close(void);
+    NS_IMETHOD Close();
 
     nsresult WriteObjectCommon(nsISupports* aObject,
                                PRBool aIsStrongRef,
@@ -521,7 +521,8 @@ class NS_COM nsFastLoadFileUpdater
     // nsIFastLoadFileIO methods
     NS_DECL_NSIFASTLOADFILEIO
 
-    nsresult Open(nsFastLoadFileReader* aReader);
+    nsresult   Open(nsFastLoadFileReader* aReader);
+    NS_IMETHOD Close();
 
     static PLDHashOperator PR_CALLBACK
     CopyReadDocumentMapEntryToUpdater(PLDHashTable *aTable,